This invention relates generally to laser apparatus and particularly to a matched index laser device for generating high input powers and which minimizes surface reflection losses between adjacent elements of the device.
Laser devices which emit high energy pulses are well-known in the art. Optical ranging systems employing laser techniques within a marine environment provide useful applications for a laser device having high pulse repetition rates and high average power along with improved efficiency and reduced beam divergence. The laser output is typically only a small percentage of the input energy, a large percentage of the energy ending up as heat in the laser medium. In high input power laser devices, one or more surfaces of the active laser medium is cooled in order to extract the excess heat. Because of the limited thermal conductivity of the laser medium, the excess heat is more rapidly removed from near the surface than from the interior. Various means and methods exist for the cooling of laser apparatus designed to operate at high pulse repetition rates and high input powers. In most cases, there are but two major areas of potential energy loss within a laser device. One of the high energy loss areas which has been addressed by corrective systems of the prior art involves the thermal lensing and distortions generated within a laser medium due to the relatively high heat buildup therein. The other of the high energy loss areas involves surface reflector or Fresnel losses caused by the differing indices of refraction between adjacent element interfaces within the laser device. The thermal lensing and distortion situation is remedied in one prior art system wherein the major surfaces of laser elements are placed in physical contact with a flowing coolant having an index of refraction near that of the laser elements. For example, the laser device exhibited a refractive index of 1.5 and was matched to a cooling liquid in the nature of dimethyl sulfoxide having a refractive index of 1.48 and which performed adequately as the cooling medium. This system also has the added benefit of eliminating Fresnel losses by index matching of the adjacent element interfaces. A problem arises, however, when a laser medium is utilized having an index of refraction substantially greater than 1.5 such as either ruby at 1.76 or Neodymium:YAG at 1.82. The foregoing relatively high refractive indices would therefore require a matched index cooling medium having a similarly high index of refraction and which would, concurrently, be both optically clear and thermally conductive. The scarcity of such high refractive index cooling mediums inhibits the efficient cooling of laser mediums having high refractive indices by means of index matching techniques due to the more common coolants having indices of 1.5 and lower.